Ellipsometry is a versatile optical technique that has applications in many different fields, from the microelectronics to semiconductor industries (for characterizing oxides or photoresists on silicon wafers, for example) to biology. This very sensitive measurement technique provides unequalled capabilities for thin film metrology. As an optical technique, spectroscopic ellipsometry is nondestructive and uses polarized light to probe the dielectric properties of a sample. An ellipsometer is a type of spectrometer that measure changes in the ellipticity of polarized light upon reflection from a surface. Changes in ellipticity of an incident beam polarized light can provide information regarding properties of thin films.
The semiconductor industry is presently developing photolithographic methods utilizing 157 nm wavelength laser light as the next step in the continuing reduction of device length scales. These wavelengths lie within a region known as the vacuum ultraviolet (VUV), in which the high absorption coefficients of oxygen lower the attenuation length in standard air to fraction of a millimeter. In particular, UV light with wavelengths less than about 180 nm may be strongly absored in air due to the presence of oxygen and/or water vapor. Achieving the transmission and stability necessary for a VUV optical metrology tool, in which the optical paths are about 0.5-2 m, therefore requires oxygen concentration in the low parts-per-million (ppm) range averaged over the optical paths. A major engineering challenge in the development of optical metrology is simultaneously providing high wafer through-put and low optical absorption. Nitrogen purge chambers may be used in conjunction with VUV Spectroscopic Ellipsometer (SE) systems to reduce levels of oxygen and/or water vapor in the optical beam paths.
It is within this context that embodiments of the present invention arise.